The present invention generally relates to a method and an apparatus for countercurrent contact between a liquid to be purified and a bed of solid purifying material. More particularly, the present invention relates to a method and an apparatus for the separation of suspended solids out of the liquid by passing the liquid through the bed of purifying material in the form of solid particles.
The U.S. Pat. No. 3,512,640, patented on May 19, 1970, discloses a method and an apparatus for purifying a liquid, such as sewage or industrial liquid waste, by passing the liquid upwardly through a bed of purifying material within a liquid treating column, withdrawing a purified liquid from the bed before it has reached the uppermost surface of the bed, and pumping some of the withdrawn liquid onto the top of the bed to avoid any possible fluidization and/or expansion of the purifying particles forming the bed.
The U.S. Pat. No. 3,512,640 further discloses the possibility of replenishing some of the purifying material, which has already been soiled by suspended solids contained in the liquid being treated or purified and which is located particularly in the region of entry of the liquid into the treating column, with a fresh purifying material, the amount of the latter corresponding to the amount of the some of the purifying material. For this purpose, the apparatus is designed so that the purifying material, which has been soiled by the suspended solids in contact with the liquid being treated, can be discharged from the bottom of the treating column on one hand and a fresh purifying material can be supplied into the treating column from above and onto the uppermost surface of the bed in an amount corresponding to the amount of the purifying material discharged.
With this apparatus, the supply of the liquid to be purified must be interrupted during the entire period of time in which the discharge of the soiled purifying material takes place. Moreover, in order to effect the discharge of the soiled purifying material from the bottom of the treating column, a pressure differential must be created between the bed, that is, the interior of the liquid treating column, and the external region, that is, the inside of an exhaust piping extending from the bottom of the liquid treating column so that the soiled particles can move out of the treating column from the bottom thereof into the exhaust piping where the lower pressure is created relative to the pressure in the liquid treating column.
Because of the necessity of the pressure differential to be established for facilitating the discharge of the soiled purifying material during the replenishment, the supply of the fresh purifying material and the discharge of the soiled purifying material must be effected at a different time, that is, the supply of the fresh purifying material is followed by the discharge of the soiled purifying material.
On the other hand, during normal operation of the apparatus, some of the purified liquid, which has been drawn out of the bed from a liquid outlet defined in the treating column at a position substantially intermediate of the height of the bed, must be recycled into the treating column and onto the uppermost surface of the bed so that the recycled liquid, while flowing downwards through an upper portion of the bed above the level of the liquid outlet, can serve as a hydraulic means to prevent fluidization of the bed particles, thereby maintaining the bed in a compact condition. The amount of the recycled liquid is usually twice the amount of the liquid to be purified which is supplied upwardly through a lower portion of the bed below the level of the liquid outlet.
In view of the foregoing, not only is the apparatus of the above described U.S. patent complicated, but also the purifying capacity is limited to a relatively low value.
Furthermore, the above described U.S. patent discloses neither the employment of a regenerating device for regenerating, i.e., washing, chemically rinsing or heat-treating to refresh, the soiled purifying particles for reuse nor the employment of the regenerating device in combination with the liquid treating column.
Apart from the liquid treating column of a particular construction which may be employed in the present invention, there is well known a regenerating device which comprises a washing column having a supply opening at its top and a discharge port at its bottom. This known washing column is designed so that a batch of purifying material supplied into the washing column through the supply opening can be fluidized by a washing or rinsing liquid supplied into the washing column from the bottom thereof and flowing upwards through a bed of the purifying material to be washed. By positively fluidizing the purifying material within the washing column, suspended solids sticking to particles of the purifying material can be separated therefrom by the friction among the purifying particles being fluidized.
However, it is believed that combination of the liquid treating column of a construction disclosed in the above mentioned U.S. patent with the known washing column of the construction described above does not make the present invention obvious.